\documentclass{article}
\usepackage[paperwidth=29.7cm, paperheight=44cm, landscape,
left=0cm, right=0cm,
top=0cm, bottom=0cm]{geometry}
\usepackage{graphicx}
\usepackage{calc}
\usepackage{pdfpages}

\pagestyle{empty}
%% length and commands for calculate spine thick and offset we need
\newlength{\totalthick}
\newlength{\pagedelta}
\newlength{\pageoffset}
\newcommand{\addtothick}[2]{%
  \addtolength{\totalthick}{0.001346mm * #1 *#2}}
%
\newcommand{\updateoffset}{%
  \setlength{\pagedelta}{\maxof{\totalthick - 15pt}{0pt} / 2}%
  \setlength{\pageoffset}{\maxof{\totalthick - 15pt}{0pt} / 4 + 7.5pt}}
\begin{document}

%% use \addtothick to tell me the paper and pages we have,
%% e.g, we have 45 sheets 80g/m^2 paper (which is 90 pages in fact),
%%    and 10 sheets 100g/m^2 paper:
\addtothick{80}{45}
\addtothick{100}{10}

%% calculate the offset we need for spine and cover page
\updateoffset

%% last merge pages
\includepdfmerge[nup=2x1, noautoscale, offset=\pageoffset{} 0,
delta=\pagedelta{} 0, pages={last-last,1}]{spine.pdf, ../main.pdf, 1 }

\end{document}
